1. Field of the Invention
The present invention relates generally to an improved data processing system, and in particular, to improving the reading and writing of data. Still more particularly, the present invention relates to a system, and computer usable program code for a dual mode reader writer lock (DML).
2. Description of the Related Art
Processes executing in a data processing system read and write data associated with a variety of resources. A data file, a memory location, and an address on a disk are some examples of such a resource.
When a process executing in a data processing system has to read or write data from or to a resource, the operating system has to sequence and control the read/write requests. This control is important in ensuring that read and write requests for the same resource by multiple processes do not compromise the integrity of the data of that resource. For example, a process should not be allowed to read data while another process is writing that data. Multiple processes may be allowed to read the same data simultaneously but not in conjunction with one or more processes attempting to write that data.
To implement these and other rules for reading and writing data, operating systems implement locks. A read write lock is a data structure whose data has to be set or reset, or incremented or decremented before a process can read or write the data of an associated resource. Such a lock is also known as a reader writer lock (RWL) or a complex lock.
Setting or incrementing a RWL is called acquiring or getting a lock. Resetting or decrementing a RWL is called releasing or freeing a lock. A reader lock or a read lock is a state of a RWL that permits a process or thread to read the resource associated with the RWL. Multiple processes or threads can concurrently acquire a read lock on a common resource. A writer lock or a write lock is a state of a RWL that permits a thread to write data to the resource associated with the RWL. Only a single thread can acquire a single write lock on the resource at any given time, and no thread can hold or acquire a read lock while another thread holds a write lock on the resource.
Typically, a RWL is implemented using a single data word. A data word is a specific number of bits that are handled together in the architecture of an operating system. In one case, the size of a data word may be the number of bits that can be held in a single register in a particular processor of the data processing system. In another case, the data word size may be the minimum number of bits can be transferred from a given memory to a given processor. Commonly available operating systems have implemented data word sizes of sixteen, thirty two, sixty four, and one hundred and twenty eight bits. A data word may be of any size suitable for a particular implementation.
When an operating system manipulates a RWL, that manipulation must not be interrupted for the integrity of the lock. In other words, all the instructions for setting or resetting, or incrementing or decrementing a RWL must be executed from start to finish without being interrupted or preempted by another process. An operation whose set of instructions must be executed in this manner is called an atomic operation. Manipulation of a RWL is an atomic operation. Acquiring a lock and releasing a lock are examples of lock manipulations performed as atomic operations.
At any given time during the operation of a data processing system, several processes or threads may wish to acquire a lock for a common resource in close temporal proximity of one another. Several other processes or threads that have already acquired the lock may wish to release the lock at or near the same time. Some processes or threads wishing to acquire or release the lock may deal with read locks on the resource, while others may deal with write locks.